Parabolic trough solar fields are used to collect radiation from the sun for conversion to a usable form of energy. The solar fields include parabolic trough reflectors which are mirrored surfaces configured to track the sun's movement across the sky and concentrate solar radiation onto a solar receiver. The mirrored surfaces reflect the sun's rays toward the solar receiver, also referred to as a heat collection element (HCE), maintained at a focal point of the reflector's parabolic shape. A heat transfer fluid flows through the HCE and is heated by the reflected and concentrated radiation. The fluid is then delivered to a heat transfer system and converted to electricity or other usable form of energy.
One problem area for these systems is the configuration of the solar receiver. The receiver includes a continuous metallic heat transfer tube surrounded by an evacuated cylindrical glass envelope, wherein the tubes are joined end to end. However, the glass envelopes are typically provided in shorter segment lengths and are not contiguous. Due to the extreme temperature variations and different coefficients of thermal expansion between the metal tube and glass envelope materials, a significant number of structural failures occur at the glass-metal interface. Known configurations use a bellows-type interface to maintain a desired seal and spacing between the transfer tube and the glass envelope. However, current bellow designs remain prone to failure at the glass-metal seal interface.
Known solutions include the use of shielding devices or mirrors to minimize heat at the interface. Shielding can be effective, but it does not always protect the interface when the sun is at an extreme angular relationship to the receiver. Although somewhat effective, the overall efficiency is reduced as the shields block sunlight from being directed to the receiver. Use of a mirror-type shield is somewhat more effective but is still lacking in that the re-directed light does not angularly adjust with the movement of the sun. As a result, some solar radiation is deflected away from the heat transfer tube and wasted. Therefore, there is a need in the art to more efficiently direct light away from the glass-metal interface so as to prevent failure thereof, and to improve the overall efficiency of the parabolic trough solar collector.